Compound semiconductor structure including an epitaxial perovskite layer and method for fabricating semiconductor structures and devices

ABSTRACT

High quality epitaxial layers of monocrystalline perovskite materials ( 18 ) can be grown overlying monocrystalline substrates ( 12 ) such as gallium arsenide wafers by forming a metal template layer ( 16 ) on the monocrystalline substrate. The structure includes a metal-containing layer ( 16 ) to mitigate unwanted oxidation of underlying layers and a low-temperature seed layer ( 19 ) that prevents degradation of an epitaxial layer ( 14 ) during growth of the perovskite layer ( 18 ).

FIELD OF THE INVENTION

This invention relates generally to semiconductor structures and devicesand to a method for their fabrication, and more specifically to compoundsemiconductor structures and devices and to the fabrication and use ofcompound semiconductor structures, devices, and integrated circuits thatinclude an epitaxial perovskite layer.

BACKGROUND OF THE INVENTION

Semiconductor devices often include multiple layers of conductive,insulating, and semiconductive layers. Often, the desirable propertiesof such layers improve with the crystallinity of the layer. For example,the electron mobility and electron lifetime of semiconductive layersimprove as the crystallinity of the layer increases. Similarly, the freeelectron concentration of conductive layers and the electron chargedisplacement and electron energy recoverability of insulative ordielectric films improve as the crystallinity of these layers increases.

For many years, attempts have been made to grow various monocrystallineoxides on compound semiconductor materials such as gallium arsenide(GaAs). For example, gadolinium oxide (Gd₂O₃) has been grown overlyingGaAs using ultra high vacuum e-beam techniques. Although epitaxial oxidelayers can be formed overlying GaAs using this technique, the films aretypically of relatively poor quality and therefore can only by grown toa limited thickness. Similarly, nickel oxide (NiO) and magnesium oxide(MgO) have been epitaxially grown on GaAs, but these oxide layers tendto turn polycrystalline when grown beyond a relatively thin layer. Theattempts to grow monocrystalline oxides over compound semiconductormaterials have generally been unsuccessful because lattice mismatchesbetween the host crystal and the grown oxide have caused the resultinglayer of monocrystalline material to be of low crystalline quality.

If a large area thin film of high quality monocrystalline oxide materialwere available at low cost, a variety of semiconductor devices couldadvantageously be fabricated using the material, taking advantage of thesuperior film properties resulting from the monocrystalline structure.In particular, the dielectric, optical, magnetic, chemical,piezoelectric and similar properties of a material generally improve asthe crystallinity of the material increases.

Accordingly, a need exists for a semiconductor structure that provides ahigh quality monocrystalline film or layer over another monocrystallinematerial and for a process for making such a structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures, in which like referencesindicate similar elements, and in which:

FIG. 1 illustrates schematically, in cross section, a device structurein accordance with an exemplary embodiment of the invention;

FIG. 2 illustrates graphically the relationship between maximumattainable film thickness and lattice mismatch between a host crystaland a grown crystalline overlayer;

FIG. 3 illustrates a process for forming a structure in accordance withthe present invention;

FIGS. 4-7 illustrate intermediate structures formed during the processillustrated in FIG. 3;

FIGS. 8-10 illustrate RHEED diffraction patterns of a strontium titanatefilm;

FIGS. 11-13 illustrate RHEED diffraction patterns of a barium titanatefilm;

FIG. 14 illustrates a high resolution Transmission Electron Micrographof a structure including a monocrystalline strontium titanate film;

FIG. 15 illustrates a high resolution Transmission Electron Micrographof a structure including a monocrystalline barium titanate film; and

FIG. 16 illustrates an x-ray diffraction spectrum of a structureincluding a single crystal film of strontium titanate and bariumtitanate.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically, in cross section, a portion of asemiconductor structure 10 in accordance with an embodiment of theinvention. Semiconductor structure 10 includes a monocrystallinesubstrate 12, an epitaxial material layer 14 comprising amonocrystalline material, a metallic template layer 16, and a perovskitematerial layer 18. In this context, the term “monocrystalline” shallhave the meaning commonly used within the semiconductor industry. Theterm shall refer to materials that are a single crystal or that aresubstantially a single crystal and shall include those materials havinga relatively small number of defects such as dislocations and the likeas are commonly found in substrates of silicon or germanium or mixturesof silicon and germanium and epitaxial layers of such materials commonlyfound in the semiconductor industry.

In accordance with one embodiment of the invention, structure 10 alsoincludes a seed layer 19 between template layer 16 and perovskitematerial layer 18. As will be explained more fully below, the templateand seed layers help to initiate the growth of the monocrystallineperovskite material layer overlying substrate 12.

Substrate 12, in accordance with an embodiment of the invention, is amonocrystalline compound semiconductor wafer, as used in compoundsemiconductor device manufacturing. Exemplary compound semiconductormaterials include Group IIIA and VA elements (III-V semiconductorcompounds), mixed III-V compounds, Group II (A or B) and VIA elements(II-VI semiconductor compounds), mixed II-VI compounds, Group IV and VIelements (IV-VI semiconductor compounds), and mixed IV-VI compounds.Examples include gallium arsenide (GaAs), gallium indium arsenide(GaInAs), gallium aluminum arsenide (GaAlAs), indium phosphide (InP),cadmium sulfide (CdS), cadmium mercury telluride (CdHgTe), zinc selenide(ZnSe), zinc sulfur selenide (ZnSSe), lead selenide (PbSe), leadtelluride (PbTe), lead sulfide selenide (PbSSe), gallium nitride (GaN),Indium arsenide (InAs), indium antimonide (InSb), and the like.

Epitaxial material layer 14 is a monocrystalline material layer that isepitaxially grown overlying substrate 12. In accordance with variousembodiments of the invention, layer 14 comprises the same material assubstrate 12. By way of particular example, when substrate 12 comprisesGaAs, layer 14 is an epitaxially layer comprising GaAs grown overlyingsubstrate 12. In this case where substrate 12 and layer 14 are bothGaAs, the surface of layer 14 can be terminated with either Ga or withAs to provide a surface for subsequent epitaxial growth. Growingepitaxial material layer 14 overlying substrate 12 provides a smooth,relatively contaminant free surface for subsequent processing. Althoughlayer 14 can be grown to any desired thickness, layer 14 is preferablymore than about 100 nm thick.

Suitable layer 16 materials chemically bond to the surface of theepitaxial layer 14 at selected sites and provide sites for thenucleation of the epitaxial growth of seed layer 19. In accordance withvarious exemplary embodiments of the invention, template layer 16includes material from epitaxial layer 14 and deposited materialselected to reduce or eliminate oxidation of material layer 14. Forexample, when layer 14 comprises GaAs, titanium may be deposited ontothe GaAs surface to form a template layer including Ti—Ga, Ti—As,Ti—Ga—O, Ti—As—O, or any combination thereof. Other materials suitablefor template layer 16 formation include zirconium, hafnium, aluminum,and cobalt. An exemplary thickness of template layer 16 ranges fromabout 0 to about 10 monolayers and preferably about 0.5 to about 2monolayers.

Forming template layer 16 of a material that prevents or at leastreduces oxidation of material of layer 14 is important for severalreasons. In the specific case of GaAs, GaAs readily oxidizes, whetherthe layer is terminated with gallium or arsenic. An oxidized GaAssurface is amorphous, and thus inhibits or prevents epitaxial growthover the underlying material layer. Deposition of a metal such astitanium is used to form a template for subsequent material layergrowth, while preventing undesired oxidation of layer 14.

Seed layer 19 is preferably formed of the same material used to formperovskite material layer 18. Both seed layer 19 and material layer 18include a monocrystalline perovskite material (e.g., an oxide or nitridematerial) selected for its crystalline compatibility with underlyingsubstrate 12. Materials that are suitable for the layers 18 and 19include metal oxides such as the alkaline earth metal titanates,alkaline earth metal zirconates, alkaline earth metal hafnates, alkalineearth metal tantalates, alkaline earth metal ruthenates, alkaline earthmetal niobates, alkaline earth metal vanadates, alkaline earth metaltin-based perovskites, lanthanum aluminate, lanthanum scandium oxide,and other perovskite oxide materials. Additionally, various nitridessuch as gallium nitride, aluminum nitride, and boron nitride may also beused for the layer 18. Most of these materials are insulators, althoughstrontium ruthenate, for example, is a conductor. Generally, thesematerials are metal oxides or metal nitrides, and more particularly,these metal oxides or nitrides typically include at least two differentmetallic elements. In some specific applications, the metal oxides ornitrides may include three or more different metallic elements.

The following non-limiting, illustrative examples illustrate variouscombinations of materials useful in structure 10 in accordance withvarious alternative embodiments of the invention. These examples aremerely illustrative, and it is not intended that the invention belimited to these illustrative examples.

EXAMPLE 1

In accordance with one embodiment of the invention, monocrystallinesubstrate 12 is a gallium arsenide substrate (001) oriented. Thesubstrate can be, for example, a gallium arsenide substrate as iscommonly used in making gallium arsenide integrated circuits having adiameter of about 50-150 mm. In accordance with this embodiment of theinvention, perovskite material layer 18 and seed layer 19 aremonocrystalline layers of Sr_(z)Ba_(1−z)TiO₃ where z has a value between0 and 1. The lattice structure of the resulting crystalline oxideexhibits a substantially 45 degree rotation with respect to thesubstrate lattice structure. The perovskite material layer can have athickness of about 0.2 to about 100 nanometers (nm) and preferably has athickness of about 5 nm, depending on the desired electrical and opticalproperties of the layer. In this example, template layer 16 comprisesabout 0.5 monolayer of titanium.

EXAMPLE 2

In accordance with a further embodiment of the invention,monocrystalline substrate 12 comprises compound semiconductor materialsin an indium phosphide (InP) system. In this system, the compoundsemiconductor material can be, for example, indium phosphide (InP),indium gallium arsenide (InGaAs), aluminum indium arsenide, (AlInAs), oraluminum gallium indium arsenic phosphide (AlGaInAsP). In this case,perovskite material layer 18 is a monocrystalline oxide of strontium orbarium zirconate or hafnate in a cubic or orthorhombic phase. Layer 18can have a thickness of about 0.2-100 nm and preferably has a thicknessof at least 4 nm to ensure adequate crystalline and surface quality andis formed of monocrystalline SrZrO₃, BaZrO₃, SrHfO₃, BaSnO₃ or BaHfO₃.The lattice structure of the resulting crystalline oxide exhibits asubstantially 45 degree rotation with respect to the substrate latticestructure.

A suitable template for this structure is about 0-10 monolayers of oneof a material M-N and a material M-O-N, wherein M is selected from atleast one of Zr, Hf, Ti, Sr, and Ba; and N is selected from at least oneof As, P, Ga, Al, and In. Alternatively, the template may comprise 0-10monolayers of zirconium-arsenic (Zr—As), zirconium-phosphorus (Zr—P),hafnium-arsenic (Hf—As), hafnium-phosphorus (Hf—P), and preferably 0.5-2monolayers of one of these materials. The resulting lattice structure ofthe perovskite material layer exhibits a substantially 45 degreerotation with respect to the substrate lattice structure and a latticemismatch between the substrate and the perovskite material layer of lessthan 2.5%, and preferably less than about 1.0%.

EXAMPLE 3

In accordance with a further embodiment of the invention, a structure isprovided that is suitable for the growth of an epitaxial perovskite filmoverlying a substrate comprising a II-VI material. Exemplary II-VIcompound semiconductor materials include zinc selenide (ZnSe) or zincsulfur selenide (ZnSSe).

In accordance with this example, perovskite layer 18 isSr_(x)Ba_(1-x)TiO₃, where x has a value between 0 and 1, and has athickness of about 2-100 nm. The lattice structure of the resultingcrystalline oxide exhibits a substantially 45 degree rotation withrespect to the substrate lattice structure. A suitable template for thismaterial system includes 0-10 monolayers of zinc-oxygen (Zn—O) followedby 0.5-2 monolayers of an excess of zinc followed by the selenidation ofzinc on the surface. Alternatively, a template can be, for example,0.5-10 monolayers of strontium-sulfur (Sr—S) followed by the ZnSSe.

Referring again to FIG. 1, substrate 12 is a compound semiconductorsubstrate such as a monocrystalline gallium arsenide substrate. Thecrystalline structure of the monocrystalline substrate is characterizedby a lattice constant and by a lattice orientation. As used herein,lattice constant refers to the distance between atoms of a cell measuredin the plane of the surface. In similar manner, perovskite materiallayer 18 is also a monocrystalline material and the lattice of thatmonocrystalline material is characterized by a lattice constant and acrystal orientation. The lattice constants of layer 18 andmonocrystalline substrate 12 must be closely matched or, alternatively,must be such that upon rotation of one crystal orientation with respectto the other crystal orientation, a substantial match in latticeconstants is achieved. In this context the terms “substantially equal”and “substantially matched” mean that there is sufficient similaritybetween the lattice constants to permit the growth of a high qualitycrystalline layer on the underlying layer.

FIG. 2 illustrates graphically the relationship of the achievablethickness of a grown crystal layer of high crystalline quality as afunction of the mismatch between the lattice constants of the hostcrystal and the grown crystal. Curve 20 illustrates the boundary of highcrystalline quality material. The area to the right of curve 20represents layers that have a large number of defects. With no latticemismatch, it is theoretically possible to grow an infinitely thick, highquality epitaxial layer on the host crystal. As the mismatch in latticeconstants increases, the thickness of achievable, high qualitycrystalline layer decreases rapidly.

In accordance with one embodiment of the invention, substrate 12 is a(001) oriented monocrystalline gallium arsenide wafer and perovskitematerial layer 18 is a layer of strontium barium titanate (Sr,Ba)TiO₃.Substantial (i.e., effective) matching of lattice constants betweenthese two materials is achieved by rotating the crystal orientation ofthe titanate material by approximately 45° with respect to the crystalorientation of the gallium arsenide substrate wafer. As a result, inaccordance with an embodiment of the invention, a high quality, thick,monocrystalline titanate layer 18 is achievable.

FIGS. 3-7 illustrate a process 30, in accordance with one embodiment ofthe invention, for fabricating a semiconductor structure such as thestructure depicted in FIG. 1. The process starts by providing amonocrystalline semiconductor substrate 12 (step 32), as illustrated inFIGS. 3 and 4. In accordance with a preferred embodiment of theinvention, the semiconductor substrate is a gallium arsenide waferhaving a (100) orientation. At least a portion of the semiconductorsubstrate has a bare surface, although other portions of the substrate,as described below, may encompass other structures. The term “bare” inthis context means that the surface in the portion of the substrate hasbeen cleaned to remove any oxides, contaminants, or other foreignmaterial. As is well known, bare gallium arsenide is highly reactive andreadily forms a native oxide. The term “bare” is intended to encompasssuch a native oxide.

In order to epitaxially grow a monocrystalline oxide layer overlying themonocrystalline substrate, the native oxide layer must first be removedto expose the crystalline structure of the underlying substrate (step34). The following process is preferably carried out by molecular beamepitaxy (MBE), although other epitaxial processes may also be used inaccordance with the present invention. The native oxide can be removedby heating substrate to a temperature of about 550° C. to about 620° C.and preferably to about 580° C. in the presence of an arsenic flux.

It is understood that precise measurement of actual temperatures in MBEequipment, as well as other processing equipment, is difficult, and iscommonly accomplished by the use of a pyrometer or by means of athermocouple placed in close proximity to the substrate. Calibrationscan be performed to correlate the pyrometer temperature reading to thatof the thermocouple. However, neither temperature reading is necessarilya precise indication of actual substrate temperature. Furthermore,variations may exist when measuring temperatures from one MBE system toanother MBE system. For the purpose of this description, typicalpyrometer temperatures will be used, and it should be understood thatvariations may exist in practice due to these measurement difficulties.

Following the removal of the oxide from the surface of the substrate, inaccordance with one embodiment of the invention, the substrate is cooledto a temperature in the range of about 300-600° C., preferably about550° C., and an epitaxial layer of GaAs 14 is grown on substrate 12layer by molecular beam epitaxy (step 36). The MBE process is initiatedby opening the shutter in the MBE apparatus to expose gallium in thepresence of arsenic flux. The surface of layer 14 can terminate witheither gallium (4×2, 6×2, or 3×2 surface structure) or arsenic (2×4 or4×4 surface structure). Epitaxial layer 14 provides a relatively smooth,well-constructed, contaminant free surface for subsequent processing.Step 36 may be carried out in the same reaction apparatus used toperform step 34 (e.g., an MBE apparatus) or another apparatus.

Template layer 16 is formed by depositing about 0.5 to about 2monolayers of titanium onto the surface of layer 14 (step 38), asillustrated in FIG. 6. In the case where titanium is the depositedmetal, step 38 is carried out at a temperature of about 25° C. to about500° C. when layer 14 terminates with arsenic and about 25° C. to about300° C. when layer 14 terminates with gallium. Template layer 16includes the deposited titanium and may additionally include arsenic,gallium, and/or oxygen.

Step 38 may be performed using the same apparatus used for steps 34-36or another apparatus. If step 38 is performed in another apparatus,layer 14 is preferably capped with arsenic to mitigate surfacecontamination and/or transferred in under vacuum to preserve theintegrity of the surface.

Seed layer 19, having a thickness of about one to several monolayers, isepitaxially formed overlying layer 16 (step 40). Step 40 is preferablycarried out at relatively low temperature—e.g., from about 25° C. toabout 400° C., and preferably about 300° C. to about 350° C. at apartial pressure of molecular oxygen of about 10⁻⁸ to about 10⁻⁵ Torr toprevent unwanted degradation of layer 14 and/or substrate 12.Alternatively, the process can be carried out in an activated oxygenenvironment.

After seed layer 19 is at least partially formed, the structure isheated to a temperature of about 500 to about 620° C. and preferablyabout 550° C. under ultra high vacuum conditions to anneal layer 19 toprovide a high-quality crystalline layer for subsequent processing. Thecrystalline quality of layer 19 can be monitored, for example, usingin-situ RHEED diffraction analysis techniques. FIGS. 8-10 illustrateRHEED diffraction patters of a SrTiO₃ film. The sharp diffractionstreaks in the [210], [110], and/or [100] directions indicate a smoothsurface and a high degree of crystallinity of the SrTiO₃ film. FIGS.11-13 illustrate similar RHEED diffraction patterns for a BaTiO₃ film.

Once seed layer 19 is formed, layer 18 (illustrated in FIG. 1) is grownat a relatively higher temperature—e.g., about 300° C. to about 620° C.with at a partial pressure of molecular oxygen of about 10⁻⁸ to about10⁻⁵ Torr (step 44). If the deposition temperature is less than about500° C., it may be desirable to periodically anneal layer 18, asdiscussed about in connection with step 42.

FIG. 14 is a high resolution Transmission Electron Micrograph (TEM) ofmaterial manufactured in accordance with one embodiment of the presentinvention, illustrating high-quality crystalline growth of strontiumtitanate layer 18 overlying gallium arsenide substrate 12 and FIG. 15 isa high resolution TEM, illustrating high-quality barium titanateoverlying a GaAs substrate. FIG. 16 illustrates an x-ray diffractionspectra taken on a structure including SrTiO₃ and BaTiO₃ monocrystallinelayers 18 grown on gallium arsenide substrates 12. The peaks in thespectrum indicate that both layers 18 and substrates 12 aremonocrystalline.

The process described above illustrates a process for forming asemiconductor structure including a gallium arsenide substrate and anoverlying monocrystalline oxide layer by the process of molecular beamepitaxy. The process can also be carried out by the process of chemicalvapor deposition (CVD), metal organic chemical vapor deposition (MOCVD),migration enhanced epitaxy (MEE), atomic layer epitaxy (ALE), physicalvapor deposition (PVD), chemical solution deposition (CSD), pulsed laserdeposition (PLD), or the like.

Clearly, those embodiments specifically describing structures havingcompound semiconductor portions and oxide material portions are meant toillustrate embodiments of the present invention and not limit thepresent invention. There are a multiplicity of other combinations andother embodiments of the present invention. For example, the presentinvention includes structures and methods for fabricating materiallayers which form semiconductor structures, devices and integratedcircuits including other layers such as metal and non-metal layers.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the present invention as set forthin the claims below. Accordingly, the specification and figures are tobe regarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeatures or elements of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1. A process for fabricating a semiconductor structure comprising:providing a monocrystalline compound semiconductor substrate, thesubstrate including gallium arsenide and having a lattice structure;forming an epitaxial layer of a compound semiconductor materialoverlying the substrate, the epitaxial layer including a monocrystallinegallium arsenide material with a surface terminating with either galliumor arsenic, wherein the gallium terminated surface has one of a 4×2,6×2, or 3×2 surface structure, and wherein the arsenic terminatedsurface has one of a 2×4, or 4×4 surface structure, the epitaxial layerproviding a smooth and contaminant free surface for subsequentprocessing; depositing, at a temperature of about 25° C. to about 300°C., a metal selected from the group consisting of titanium, zirconium,and hafnium onto the epitaxial layer to form a template layer comprisingthe metal; depositing, at a pressure of about 10⁻⁶ to about 10⁻⁵ Torrand at a temperature of about 25° C. to about 400° C., a seed layercomprising perovskite material onto the template layer; heating thestructure, at a temperature of up to about 620° C., to anneal the seedlayer until the seed layer is monocrystalline; and epitaxially growing,at a temperature of about 300° C. to about 620° C. and at a pressure ofabout 10⁻⁶ to about 10⁻⁵ Torr, a monocrystalline oxide layer overlyingthe seed layer; the oxidelayer selected from the group consisting ofalkaline earth metal titanates, alkaline earth metal zirconates,alkaline earth metal hafnates, alkaline earth metal tantalates, alkalineearth metal ruthenates, alkaline earth metal niobates, alkaline earthmetal vanadates, alkaline earth metal tin-based perovskites, lanthanumaluminate, lanthanum scandium oxide, and combinations thereof, whereinthe seed layer helps to initiate growth of the monocrystalline oxidelayer, further wherein a lattice structure of the monocrystalline oxidelayer exhibits a substantially 45 degree rotation with respect to thesubstrate lattice structure.
 2. The process of claim 1, furthercomprising the step of heating the substrate to a temperature of about550-620° C. to remove a native oxide from the surface of the substrateprior to the step of forming the epitaxial layer.
 3. The process ofclaim 1, wherein the step of depositing a metal comprises depositingabout one to about 2 monolayers of metal.
 4. The process of claim 1,wherein the step of depositing a seed layer comprises depositingSr(Ba)TiO₃.
 5. The process at claim 1, wherein the step of depositing aseed layer comprises depositing strontium titanate.
 6. The process ofclaim 1, wherein the step of depositing a seed layer comprisesdepositing barium titanate.